The present invention generally relates to a sheetline for fabrication of plastic sheet and, more particularly, to a roll reconfiguring method for making multiple adjustments in the relative positions of finishing rolls of the sheetline to optimize the surface finish quality of the plastic sheet.
Sheetlines are known which employ a plurality of finishing rolls for rolling out a web of plastic melt outputted from an extruder die to solidify the plastic melt into sheet form. For the production of plastic sheets having optical quality finish, opposite surfaces of the sheets must be polished. In order to polish the opposite sheet surfaces, one conventional practice is to pass the web of plastic melt successively through a pair of nips formed between three finishing rolls. The surfaces of the sheets are polished by contact with the highly-polished surfaces of the finishing rolls. Also, the temperatures of the rolls are controlled for causing the proper sequence of cooling and solidifying of the respective surfaces of the sheets after they contact the rolls.
One conventional configuration of the finishing rolls is a vertical stack wherein the three finishing rolls are positioned one above the other with the pair of nips defined in between. An extruder feeds the plastic melt through a sheet die having a long narrow slit and aligned with the nip between the center and upper rolls. The web of plastic melt flows from the die in the horizontal direction into the primary or first nip where the gauge thickness of the sheet is determined by the width of the nip, or distance between the upper and center rolls. After passing through the first nip, the sheet of plastic melt wraps around the center roll where its one surface in contact with the outer polished surface of the center roll is polished by it. Next, the sheet of plastic melt passes through the second nip between the center roll and the lower roll and about the surface of the lower roll where the other opposite surface of the sheet in contact with the polished surface of the lower roll is polished by it.
However, for production of plastic sheets of optimum optical quality, the sheet surfaces to be polished must actually adhere to the respective roll surfaces. Thus, the temperature of the plastic melt must be hot enough for the sheet surfaces to stick to the respective rolls. If the web of plastic melt cools to rapidly such adherence is not possible and a poor optical quality surface finish results.
One problem with plastic sheets produced by the finishing rolls configured in the vertical stack is the tendency of the web of plastic melt, being fed horizontally by the die into the primary nip, to sag due to the effect of gravity as it emerges from the die. The sagging web of plastic melt contacts the center roll upstream of the nip for a sufficient interval of time to permit premature cooling of the plastic melt and cause inadequate adherence of the sheet to the center roll as it departs the first nip.
Also, such premature cooling of the web of plastic melt from contact with the center roll upstream of the first nip prevents air bubbles in the plastic melt from being squeezed out by passage through the nip. Instead, the air bubbles stay trapped in the sheet of plastic melt and cause imperfections in the surface finish.
One approach to overcoming the above-described problems is to reconfigure the finishing rolls from the vertical stack to an inclined stack wherein at least the upper and center rolls lie along a plane inclined at approximately 45.degree.. With the web of plastic melt now feeding into the primary nip from the extruder die at a 45.degree. angle, instead of in the horizontal direction, a smaller component of the force of gravity is acting on the web and thereby the tendency for the web of plastic melt to sag is less. However, some sagging still occurs. It has also been proposed to mount the lower, or third, roll for pivotal movement about the axis of the center roll to provide various possible configurations of the lower roll with the fixed pair of 45.degree. inclined upper and center rolls.
While the 45.degree. reconfiguration of finishing rolls is an improvement over the vertical stack, such reconfiguration is not seen as the optimum solution to the problem. Consequently, a need still exists for an approach to the positioning of the finishing roll that will overcome the above-cited problems of the prior art.